1. Field of the Invention
The present invention generally relates to an air/fuel ratio sensor having one electrochemical cell, which can detect an air/fuel ratio in an exhaust gas exhausted from e.g. an automobile engine. More specifically this invention relates to an air/fuel ratio sensor using one oxygen-ion conductive cell, which can detect not only a stoichiometric air/fuel ratio (.lambda.=1), but also an air/fuel ratio in a fuel-lean region (.lambda.&gt;1).
2. Description of the Prior Art
In recent years, most of automobile engines have been equipped with an air/fuel feedback control system to run the engines in a best condition reducing harmful exhaust gas components such as CO, NOx and HC. Either of two types of air/fuel ratio sensors is placed in the exhaust gas stream in such a feed back control system; one type is a stoichiometric air-fuel ratio sensor having one oxygen-ion conductive solid electrolyte cell and the other type is a universal or rather wide range air/fuel ratio sensor having two oxygen-ion conductive solid electrolyte cells.
The stoichiometric air/fuel sensor has one cell electrolyte cell that outputs an abrupt voltage variation-across two electrodes that sandwich the cell electrolyte, when the stoichiometric air/fuel mixture(.lambda.=1) is carbureted and combusted in an internal combustion engine. The stoichometric air/fuel sensor is often called a .lambda. sensor or a .lambda. air fuel sensor. This type sensor is described, e.g. in a U.S. Pat. No. 5,518,603.
On the other hand, the universal air/fuel sensor has two electrochemical cells, featuring in that the sensor can detect not only the stoichiometric air/fuel ratio(.lambda.=1) but also a universal or rather wide range air fuel ratio other than .lambda.=1 (i.e. .lambda.&gt;1 and .lambda.&lt;1). The reason why such a wide range air fuel control can be done by this two-cell type sensor is that one of the cells is used as an oxygen pumping cell that pumps the oxygen out or into an oxygen reference chamber formed between the two cell of the sensor. If the pumping cell pumps out the oxygen from the reference chamber, it means that an air/fuel mixture in the engine is shifted to a fuel-lean region, i.e .lambda.&gt;1. If the pumping cell pumps oxygen into the reference chamber, it means that the air/fuel mixture shifted to a fuel-rich region (i.e. .lambda.&lt;1) is combusted in the engine.
A merit of using the universal sensor comprised of the two cell in such an air/fuel feedback control system is that an amount of fuel to be carbureted with the air into the engine can be linearly detected by a current flowing through the pumping cell across which a constant voltage is applied. This two-cell type sensor is disclosed, e.g. in a U.S. Pat. No. 5,194,135.